Functional traits affect the demographic performance of individuals in their environment, leading to fitness differences that scale up to drive population dynamics and community assembly. Understanding the links between traits and fitness is, therefore, critical for predicting how populations and communities respond to environmental change. However, the net effects of traits on species fitness are largely unknown because we have lacked a framework for estimating fitness across multiple species and environments. We present a modelling framework that integrates trait effects on demographic performance over the life cycles of individuals to estimate the net effect of traits on species fitness. This approach involves (1) modelling trait effects on individual demographic rates (growth, survival and recruitment) as multidimensional performance surfaces that vary with individual size and environment and (2) integrating these effects into a population model to project population growth rates (i.e., fitness) as a function of traits and environment. We illustrate our approach by estimating performance surfaces and fitness landscapes for trees across a temperature gradient in the eastern United States. Functional traits (wood density, specific leaf area and maximum height) interacted with individual size and temperature to influence tree growth, survival and recruitment rates, generating demographic trade‐offs and shaping the contours of fitness landscapes. Tall tree species had high survival, growth and fitness across the temperature gradient. Wood density and specific leaf area had interactive effects on demographic performance, resulting in fitness landscapes with multiple peaks. With this approach it is now possible to empirically estimate the net effect of traits on fitness, leading to an improved understanding of the selective forces that drive community assembly and permitting generalizable predictions of population and community dynamics in changing environments.
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Abstract Modern coexistence theory holds that stabilizing mechanisms, whereby species limit the growth of conspecifics more than that of other species, are necessary for species to coexist. Here, we used experimental and observational approaches to assess stabilizing forces in eight locally co‐occurring, annual, legume species in the genus
Trifolium . We experimentally measured self‐limitation in the field by transplantingTrifolium species into each other's field niches while varying competition and related these patterns to the field coexistence dynamics of naturalTrifolium populations. We found thatTrifolium species differed in their responses to local environmental gradients and performed best in their home environments, consistent with habitat specialization and presenting a possible barrier to coexistence at fine scales. We found significant self‐limitation for 5 of 42 pairwise species combinations measured experimentally with competitors absent, indicating stabilization through plant–soil feedbacks and other indirect interactions, whereas self‐limitation was largely absent when neighbors were present, indicating destabilizing effects of direct plant–plant interactions. The degree of self‐limitation measured in our field experiment explained year‐to‐year dynamics of coexistence byTrifolium species in natural communities. By assessing stabilizing forces and environmental responses in the fulln ‐dimensional field niche, this study sheds light on the roles of habitat specialization, plant–soil feedbacks, and plant interactions in determining species coexistence at local scales.
